Heterotactic polymers

Cyclopolymerization of the bis-methacrylates (10, ll)6" 6j or bis-styrene derivatives (12)64 has been used to produce heterotactic polymers and optically active atactic polymers. Cyclopolymcrization of racemic 13 by ATRP with a catalyst based on a chiral ligand (Scheme 8.12) gave preferential conversion of the (S, )-enantiomer. 66... 

To prove the correctness of this analysis we shall try to look for alternative explanations. A stereoregular heterotactic polymer. . . mrmrmr. . . could also explain the presence of a methyl singlet and a well-separated doublet of doublets for methylene protons, in agreement with the spectrum of the first sample in fact, there would be present the mr triad and the rmr tetrad with diastereotopic protons. However, in addition, an equal amount of the mrm tetrad should be present, yet the corresponding singlet is not visible with the required intensity. In the same way other hypothetical structures can be rejected. 

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

Knowledge of the existence of the various stereoregular forms and a comprehension of their respective structures adds both to the scientific interest and an appreciation of the physical properties and the potential utility of these materials. In general terms, atactic (heterotactic) polymers tend to be amorphous (noncrystalline) in the solid state. They will have a lower melting... 

A polymer having more than SO % porticm of heterotactic triad should be defined to be a heterotactic polymer, since the heterotactic content could attain at most to 50% even in a sterically random polymer. The formation of heterotactic polymer seems to... 

In the literatures we can find out a number of data which indicate the formation of heterotactic polymers, although the authors have not commented about it. These Include poly(methyl methacrylate) and poly(ethyl methacrylate) obtained by octyl-potassium, biphenylpotassium, fluorenylpotassium (Table 5), benzylcal-dum or potassium alkoxide in polar solvent, and poly(phenyl methacrylate) prepared by n-, iso- and sec-butylmagnesium bromide in toluene at low temperature (Table S). These clearly indicate that not only the size of the substituent of monomer but also the nature of the counter cation or the pdymerization medium is important factor for the formation of heterotactic polymer. 

The large majority of stereoselective acrylate polymerizations afford either isotactic or syndiotactic-biased material and since heterotactic polymers have only been prepared on rare occasions, they are not covered in depth in this chapter. The principal initiating system for the production of heterotactic poly(alkyl methacrylates) is a stoichiometric 1 1 mixture of tcrt-butyllithium and bis(2,6-di-tert-butylphenoxy)methylaluminum. The heterotactic content is particularly pronounced at low temperatures and increases in the order methyl < ethyl < propyl < allyl methacrylates. For example, at -95 °C, poly(allyl methacrylate) with 95.8% mr triad content is obtained after reduction to poly(methacrylic acid) and subsequent methylation, highly heterotactic PMMA results. ... 

Carpentier and coworkers recently synthesized yttrium and lanthanide LA polymerization catalysts with tetradentate alkoxy-amino-bis(phenolate) ligands (30-32, Figure 25.14). Yttrium compounds 30 and 31 polymerized rac-LA to give predominantly heterotactic polymer (Ph = 0.8) in THF, showing less selectivity in toluene (Ph = 0.6). The heterotacticity of PLA formed by lanthanide complex 32 from rac-LA in THF is significantly decreased (Ph = 0.64) compared to its yttrium analogue 31. 

In the case of chiral cyclic monomers the prepared polymer is isotactic, while this is not a necessity in principle in the case of chiral olefins. In most of the cases however, the choice is not as perfect as defined and, generally, one speaks of stereoelective process when there is a preferential polymerization of one type of enantiomer from a mixture. Moreover the sites controlling the polymerization could be more or less stereospecific, so that heterotactic polymers could in principle be obtained with the predominance of one type of enantiomeric unit. 

In the most general case, both crystalline (isotactic) and amorphous (heterotactic) polymers are formed. 

Usually free-radical polymerization of acrylic systems produces mainly stereorandom (atactic or heterotactic) polymers, but Duran and Gramain have demonstrated a correlation between tacticity triads and the spacer length in acrylate monomers subjected to free-radical solution... 

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